In U.S. Pat. No. 3,925,625 issued Dec. 9, 1975 to R. J. Angner et al, there is disclosed a key telephone line circuit which is connected in shunt across the tip and ring lead communication path between the central switching machine and the subscriber's station to control the HOLD, BUSY and IDLE conditions of the station and to provide the necessary visual and audible signaling to the stations. The line circuit disclosed in the above-identified patent operates to detect ac current flowing for ringing control, while also detecting the transient response of the system for hold and disconnect control. The transient signals are generated upon on-hook to off-hook or off-hook to on-hook circuit transitions.
Shunt control has several advantages, but the most important one is the perfect balance of the tip and ring circuitry at all times. Other advantages include the ease of application of music and tone-on-hold signals and the fact that the circuit can be removed for maintenance in any state but HOLD without disrupting customer service. The shunt principle of HOLD/DISCONNECT detection is as follows: When a key station goes on-hook, two things happen sequentially:
1. the tip and ring is disconnected at the telephone set which causes current to flow through the ring-up capacitor as it charges to -48v, and PA1 2. the A-lead is broken.
When a line is placed on HOLD, the A-lead is broken but no current flows through the ring-up capacitor until sometime later. Thus, the HOLD/DISCONNECT distinction is made by detecting the presence of current flow in the ring-up capacitor before the A-lead is broken.
Due to the nature of the switchhook contacts, the tip and ring leads open before the A-lead opens on disconnect. Since the disconnect condition is determined from a transient signal caused by the tip and ring lead transition and thus can be finished before the A-lead opens, timing is used to remember the transient signal to insure that the line circuit goes into the proper state. However, under certain conditions if the transient signal of the immediately past transition were to be forgotten, the line circuit would respond to an A-lead opening by going into the HOLD state instead of going into the desired IDLE state.
Thus a problem exists when using electronics in the presence of various preexisting mechanical pecularities to detect transient signals, which by definition are only temporary in nature.
It is a further problem to arrange a key telephone line circuit, which must interface with any number of switching machines having widely varying signaling tolerances and with mechanical telephone stations also having varying operating tolerances to take advantage of integrated electronic technology.
Further problems exist when using the transient response of the line circuit for control purposes. Primarily, the problems come about due to normal voltage pulses on the line and when other equipment is connected in parallel with the line circuit.